Organic Chemistry 1 · Radicals, alcohols and ethers
2-Methoxy-2-phenylethan-1-ol from styrene oxide in acidic methanol
Conditions: MeOH (solvent), H2SO4 (cat.)
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The mechanism, step by step
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Protonate the epoxide
Sulfuric acid protonates the epoxide oxygen; methanol is assumed to be the solvent.
The curved arrows
- A lone pair on the epoxide oxygen takes the acidic proton.
- The O–H bond pair stays on oxygen, giving hydrogen sulfate.
Where does methanol attack?
The C–O bonds of the protonated epoxide are not equally weak.
Methanol opens at the benzylic carbon
Methanol attacks the benzylic carbon, which has developed substantial cation character, as the benzylic C–O bond breaks. The attacked carbon inverts, but the substrate is racemic.
The curved arrows
- A lone pair of methanol oxygen bonds to the benzylic carbon.
- The benzylic C–O bond pair stays on oxygen, which becomes the CH₂OH group.
Hydrogen sulfate removes the proton
Hydrogen sulfate takes the proton from the new oxonium ion, giving the neutral ether and regenerating sulfuric acid.
The curved arrows
- A hydrogen sulfate oxygen lone pair takes the O–H proton.
- The O–H bond pair stays on oxygen, which becomes neutral.
2-Methoxy-2-phenylethan-1-ol
The product is 2-methoxy-2-phenylethan-1-ol, with the methoxy group on the benzylic carbon and a primary OH; sulfuric acid is regenerated.
What you'll practice
Draw the acid-catalysed opening of 2-phenyloxirane by methanol at the benzylic carbon.
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